The methods heretofore suggested for graft polymerization on hydrophobic synthetic fibers include a method utilizing a high-energy radiation, which is discussed at page 615, Part C No. 4 of the Journal of Polymer Science (1963) and in U.S. Pat. No. 3,274,295, a method utilizing low-temperature plasma which is disclosed in the U.S. Pat. No. 3,600,122, a method utilizing a persulfate which is disclosed in U.S. Pat. No. 3,297,471 and a method resorting to oxidation with ozone which is touched upon at page 753, Part C No. 4 of the Polymer Science (1963). The method which utilizes a high-energy radiation has a disadvantage that the radiation degrades the properties of the fibers under treatment. In addition, it entails an economical problem in that the cost of equipment is high. The method which utilizes low-temperature plasma necessitates a system operable under a reduced pressure and, therefore, fails to provide easy voluminous treatment from the operational point of view and proves to be economically handicapped. Besides, since the modification aimed at is limited to the very surface layer, it is difficult to obtain the effect of modification to a thorough extent. The method which utilizes a persulfate and the method which resorts to the oxidation with ozone generally provide graft polymerization of inferior efficiency and give deficient modification. Particularly in the case of fibers of high crystallinity, the efficiency of graft polymerization is extremely low so that, depending on the particular unsaturated monomer in use, the conversion of graft polymerization which is obtained at all is of a virtually negligible degree.
On the other hand, methods which accomplish the desired graft polymerization by use of a hydrophobic radical polymerization initiator have been disclosed by Japanese Patent Publication No. 502/1970 and Japanese Patent Publication No. 27743/1973. The former invention relates to a two-stage process whereby a hydrophobic synthetic polymer is subjected first to a treatment for activation with a hydrophobic radical polymerization initiator and then is reacted upon by an unsaturated monomer, whereas the latter invention concerns a single-stage process whereby a shaped article of aromatic polyester is treated in an aqueous dispersion consisting of a specific hydrophobic organic solvent, a hydrophobic radical polymerization initiator, a hydrophilic unsaturated monomer and an emulsifier. The method of the former invention is economically disadvantageous in the sense that the two-stage process calls for a highly complicated operation and consumes a considerably large amount of time. What is more, it is difficult to obtain uniform graft polymerization stably because uniformization of graft polymerization is impeded such as by the phenomenon of inactivation which occurs while the fiber under treatment is in the process of shifting from the treatment for activation to the stage of graft polymerization. The method of the latter invention, notwithstanding the advantage of one-stage process, suffers from serious defects such as lack of uniformity of graft polymerization and unsatisfactory efficiency of graft polymerization, which pose a great hindrance to the commercialization of the method.